1. Field of the Invention
The present invention relates to a power amplifying apparatus and, more particularly, to a power amplifying apparatus for a driver used in an optical disk apparatus.
2. Description of the Related Art
Power amplifiers are used in various sections of an optical disk apparatus, specifically, are used to drive driving coils for focusing and tracking control, a linear motor for an optical pickup, and a loading mechanism in an optical disk.
TC7256 (available from TOSHIBA CORP) is known as such a power amplifier. A current limiter is incorporated in this amplifier so as to prevent a current exceeding a continuous rated value from flowing through a load such as a driving coil. One end of a sensing resistor is connected to the output terminal of this amplifier, whereas the other end of the sensing resistor is connected to the load and the current limiting terminal of the current limiter. With this circuit arrangement, when an output current from the power amplifier flows through the sensing resistor, a voltage drop due to this sensing resistor occurs. As a result, a voltage corresponding to this voltage drop is applied to the current limiting terminal of the current limiter, and current limitation corresponding to this voltage is effected.
In the above-described power amplifier, however, the output of the power amplifier depends on the sensing resistor, which has a constant value. The use of this type of power amplifier is not suited for, say, focusing control, which requires an output current of a temporary excessive value in its intermittent operation.
In addition, when a loading mechanism is to be driven, a signal with logic levels is used. When this signal is at high level, a current flows through the load. Therefore, if a high-level signal continues for an excessively long period of time, an excessive load is applied to the load.